Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and an optical detector, being integrated within a pickup. The detector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to about 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm and a numerical aperture of 0.85 is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by a so-called Super-RENS structure, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super resolution layer is a nonlinear layer, which is also called a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, the reflectivity being dependent on the pit structure of a corresponding data layer. Therefore, the Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of a corresponding optical pickup. Super-RENS storage media comprising a super resolution near-field structure formed of a metal oxide are known from US 2004/0257968 and WO 2005/081242.
In “Capacity Increase in Radial Direction of Super-Resolution Near-Field Structure Read-Only Memory Disc”, Kurihara et al., Jpn. J. Appl. Phys., Vol. 46, No. 6B (2007), pp. 3898-3901, a Super-RENS disc is described, which uses a group tracking technique to reduce the track pitch between neighboring tracks below the optical resolution limit of a corresponding pickup. Experimental results are presented, which show that for tracks arranged in groups of three single tracks on the disc, an acceptable carrier-to-noise ratio for pits with a pit length of 100 nm and a track pitch of 200 nm can be obtained, by using a conventional HD-DVD pickup. A group of tracks is recognized as a single track by the far-field optics of the pickup and a conventional push-pull method could be used for tracking, which uses in addition an offset adjustment for tracking of a respective track within a group. It was experimentally verified that with a group comprising three tracks, the data capacity could be increased by 1.5-fold greater than that of compared to a conventional Super-RENS ROM-disc.
IN WO 2004/032123 a group track structure is applied to a magneto-optical read-only storage medium for providing a track pitch below the diffraction limit of a respective focusing optics. A data track within a group is selected by providing an offset value to a tracking regulation.